The size of a bankroll also depends on how good a poker player you are. If you are a cash game player that wins 60% of the time, you obviously need a bigger bankroll than a player that wins 80% of the time. Loose players experience much higher variance than tight players. Players that learn how to be mentally strong tend to experience less variance than unstable players.
If you play mtt,you need about 200~300 buy in.If you play cash game,you need about 50~100 buy in. Above all ,your bankroll should be enough to withstand fluctuations
To be honest I do not know how to increase your bankroll, as playing only in freerolls and when you receive the bankroll, I almost always lose to the next Freeroll.